Photovoltaics (PV) or solar cells are devices which convert sunlight into direct current (DC) electrical power. The most common type of solar cells are crystalline silicon cells, as opposed to cells comprised of thin films of amorphous or microcrystalline silicon and other photovoltaic materials such as CdTe, CIGS, etc. A typical crystalline silicon PV cell includes a p type silicon wafer, substrate or sheet typically less than about 0.3 mm thick with a thin layer of n-type silicon on top of a p-type region formed in a substrate. The generated voltage, or photo-voltage, and generated current by the photovoltaic device are dependent on the material properties of the p-n junction and the surface area of the device. When exposed to sunlight (consisting of energy from photons), pairs of free electrons and holes are generated in the silicon. The electric field formed across the depletion region of p-n junction separates the free electrons and holes, creating a voltage. An external circuit from n-side to p-side allows the flow of electrons when the PV cell is connected to an electrical load. Electrical power is the product of the voltage times the current generated as the electrons and holes move through an external load and eventually recombine. Given a specific size, the specific materials used, and the photovoltaic conversion efficiency of those materials, solar cells generate a specific amount of power. Individual cells are further typically tiled into modules sized to deliver a desired amount of system power. Solar modules are typically created by connecting a number of solar cells that are placed in panels with specific frames and connectors.
FIGS. 1A to C illustrate a conventional method to process silicon wafers to produce a silicon solar cell. As shown in FIG. 1A, a p-type silicon substrate 100 is doped with n-type material to form a n-type region 102 adjacent to one surface (e.g. a front surface) of the substrate. The silicon substrate 100 can be comprised of a wafer about 0.25 mm thick.
Next, typically a passivation layer 108 (e.g. SiN) is deposited on the front surface of the substrate 100. This layer also acts as an anti-reflection layer to reduce the reflection of incident photons. Next, as shown in FIG. 1B, a screen printing method is used to pattern front contacts on a front surface of substrate 100. More particularly, front contact pad 104 is patterned on the front surface using a silver/glass frit paste and screen printing techniques. For backside metallization, aluminum/glass frit paste 110 is applied to the full back surface of substrate 100 (e.g. by screen-printing or rolling).
In a next step shown in FIG. 1C, the front and back sides of the wafer are co-fired using ‘spike’ anneal to a temperature up to 850° C., causing silver to diffuse into the front side of the silicon substrate through the passivation layer and make contact to the doped n-type region 102, thereby forming contact regions 106 underlying front contact pad 104. During the spike anneal, the wafer is held, for a few seconds, slightly above the Al—Si eutectic temperature of 577° C. that causes the Al—Si alloy to form at the boundary of the paste 110 and substrate 100. On cooling, the silicon, saturated with aluminum, precipitates out and a p+ type layer 112 is formed adjacent the back surface, thereby forming the back contact and the back surface field (BSF). This field acts to repel electrons, thereby enhancing the flow of electrons in the desired current flow direction when the substrate is exposed to light and the substrate is used in a PV module.
For the back-side metallization, the problems with this conventional processes include: (1) the reflectance of the Al/Si interface is degraded due to the diffusion of Si in Al that has a considerable amount of frit, (2) the resulting aluminum film, being highly porous with entrained surface oxide, has higher resistivity and poor strength, and (3) wafer bowing related to the densification of the paste containing a high amount of frit.
Other alternatives for forming a BSF in a solar cell have been proposed. In one example, a special process is used to make the back side field using boron doped oxide. Additional prior art techniques include those described in U.S. Pat. No. 4,297,391. This describes a process of spraying high temperature conductive material on a photovoltaic cell using plasma. However, the techniques described therein project the metal particles at a very high velocity and temperature that may not be suitable for solar cell wafers.
However, there remains a need in the art for a process that can form the back contact with a back surface field in a solar cell substrate that does not suffer from problems such as those described above.